1. Field of the Invention
The present invention relates to a circuit element and a method for protecting a load circuit.
2. Description of the Related Art
An object becoming more and more important in electronics, especially under safety aspects, consists in how individual devices or circuit parts can be shut down in targeted, permanent and as-inexpensive-as-possible manner, so as to prevent greater consequential damage.
This topic is to be examined in greater detail on the basis of two embodiments according to the prior art.
Today, power semiconductors are employed to a great extent for switching electric loads, such as lamps, valves, motors, heating elements, etc., but increasingly also in the field of power management for switching off individual circuit parts, e.g. to reduce the energy consumption of battery-operated devices. The two typical arrangements of switch and load are illustrated in FIG. 12 and FIG. 13.
FIG. 12 shows a so-called low-side switch. A voltage terminal 5, a switch 10, which is embodied as a field-effect transistor here, for example, a load 20, a fuse 30, and a course of the load current 40 are illustrated.
Since the switch 10 lies in the plus line to the ground line, it is considered as a low-side switch. Here, it is important for low loss power in the switch 10 that the switch has by far lower electric resistance in the on-state than the load 20. FIG. 13 shows a high-side switch. The switch 10 here lies in the plus line to the load 20. This is the only difference as opposed to the low-side switch illustrated in FIG. 12.
It also applies to the high-side switch that the loss power in the switch 10 should be low compared with the power in the electric resistor, which here forms the load 20, which means that the resistance of the switch 10 should be low compared with the resistance of the load 20.
Power MOSFETs have largely prevailed as electronic switches for these voltage applications. In the last few years, a very rapid development toward ever-lower pass resistivities, which can be seen from the calculation Rds(on)XA, with Rds(on) representing the pass resistivity and A symbolizing the area, has made it possible that today currents of many amperes are manageable with semiconductor switches directly mounted on the circuit board without special cooling measures.
But one problem arises when complete switch-on no longer takes place through faults in the semiconductor switch or in its control. The switch then no longer reaches its low nominal pass resistance, so that the loss power in the switch also increases very strongly. In the worst case of the power matching, i.e. when the pass resistance of the switch comes into the range of the value of the load resistance, the loss power in the switch may reach up to one fourth of the nominal load power. An example can make this clearer:
In a power MOSFET with a pass resistance of 10 mΩ, which is employed as a switch for a load of 120 watts at 12 volts, a loss power of 1 watt develops in the nominal operation. The cooling of the MOSFET will be designed for this loss power in a concrete circuit.
In the embodiment of the prior art, if the pass resistance, however, increases by a fault, which may for example occur in the control, the loss power in the switch may reach values of up to 40 watts. With a cooling designed for 1 watt, this will very quickly lead to dangerously high temperatures up to a fire hazard of the circuit board, for example. Common protection elements, such as fuses 30, cannot compensate for this fault case, because no overcurrent of any kind occurs.
The load 20 always limits the current 40 to a value not exceeding the nominal operating current. Arbitrary monitoring circuits, which detect this fault, also would not be of any help here, because switch-off of the load current by means of the power switch would no longer be possible at every assumed defect in the control circuit or in the power switch.
FIG. 14 explains a further embodiment according to the prior art. The voltage terminal 5, the fuse 30, a varistor 50, a multilayer ceramic capacitor 60 and a tantalum electrolytic capacitor 70 are illustrated. FIG. 14 shows a series connection of the fuse 30 with a parallel connection including the varistor 50, the multilayer ceramic capacitor 60 and the tantalum electrolytic capacitor 70. The safety-critical devices, such as the varistor 50, the multilayer ceramic capacitor 60 and the tantalum electrolytic capacitor 70, are directly present at the supply voltage 5 via the fuse 30. In the operational state, these devices have a negligible leakage current at the nominal operating voltage, and hence a negligible static loss power. But if the leakage current increases in the fault case, or if a plate contact occurs especially in multilayer ceramic capacitors due to mechanical stress, the static loss power increases very strongly and may lead to extreme overheating of the device.
Especially in larger, centrally protected assemblies, there is the problem that although the current occurring in this fault case is sufficient to generate extreme overtemperatures with fire hazard locally, the current does not rise high enough to make the central protection element trigger, which is implemented in the fuse 30 here. The varistor 50, the multilayer ceramic capacitor 60 and the tantalum electrolytic capacitor 70 in this embodiment only symbolically represent a series of further devices very likely to become low-ohmic at the end of their life at overload or premature failure. All theses devices pose a risk with reference to the above-mentioned facts.
In this connection, a disadvantage of the prior art again becomes obvious that here an externally activable protection element is missing, which would enable separating safety-critical devices from electric voltage in targeted manner. In numerous cases, with this at least an emergency operation functionality of an assembly could be maintained.
According to the prior art, largely fuses can be employed for protection against damage by too high currents. These may also be obtained in most varied constructions and trigger characteristics. At the same time, even as overcurrent protection, there are cold conductors, such as PTC elements, finding broad application on ceramic or polymer basis, such as a poly switch™. But if such a fault case that no overcurrent occurs happens, as described above, these fuses are not suited as protection elements. These fuses are also not suited for the protection of safety-critical devices due to their constructive size, costs and trigger characteristics. In capacitors, for example, the AC operating current may lie significantly above the DC trigger current to be demanded, a requirement that cannot be met with a classic fuse.
Apart from that, in embodiments according to the prior art, also temperature sensors are employed to monitor circuits with respect to temperatures. But this kind of monitoring also cannot offer protection in the described case of a defective, no longer controllable semiconductor. The recognition of an overtemperature is of no use here, since the current flow can indeed no longer be interrupted by intervention into the control voltage of the defective switch. This also poses a high risk for the assemblies to be protected.
A further variant according to an embodiment of the prior art consists in the employment of a crowbar switch. A crowbar switch is a powerful short switch capable of triggering a present central fuse. Due to the high costs and the required space need, crowbar solutions in embodiments according to the prior art are not suitable for decentralized protective measures. A central crowbar very strongly limits the possible fields of application, since in many applications it is not tolerable to shut down the entire system instead of only a single load current path in the fault case.
U.S. Pat. No. 5,003,371 teaches a circuit element with a fuse, an input pin and an output pin. The input pin and the output pin are connected to each other via doped zones. The doped zones are arranged so that a thyristor or field-effect transistor are formed. The thyristor may be ignited by a positive signal at a gate of the field-effect transistor. Following igniting of the thyristor, the thyristor effects melting of the fuse.
U.S. Pat. No. 5,757,599 teaches a protection element in a circuit device. The protection element is connected in parallel to the circuit device. If the operating parameters of the circuit device are exceeded, the protection element causes a short of the circuit device by an irreversible short in a thyristor.
DE 19805785C1 teaches a power semiconductor module, in which a power semiconductor device is connected in electrically conducting manner via output lines. The power semiconductor module comprises interruption means irreversibly interrupting the output lines if a predetermined operating temperature threshold is exceeded. The interruption means is characterized by a volume-expanding or exploding characteristic.